1. FIELD OF THE INVENTION
The present invention relates generally to downhole drilling motors and more particularly to modifications in drilling motor construction which enable such motors to function more efficiently in bottom hole assemblies.
2. DESCRIPTION OF THE PRIOR ART
The use of motors in bore hole drilling, especially in drilling for oil and gas, has been a standard procedure in the art. Such motors are employed to rotate drill bits for boring in the earth for forming bore holes. We refer to such motors as in-hole motors when designed to be run at the end of a drill string and adjacent to the drill bit. Other components such as stabilizers, MWD sensors and pulsers are utilized with the motor and bit to form bottom hole assemblies.
There are two types of downhole motors, the hydrodynamic type known as the turbodrill downhole motor, or the positive displacement type known as the progressive cavity downhole motor.
The most common type of downhole motor is the progressive cavity type composed of a helicoidal rotor and a complimentary helicoidal stator. Such construction was first taught by Rene Moineau in U.S. Pat. No. 1,892,217 in 1932. It wasn't until 1941 when such a motor was adapted for downhole use. Such a use was taught in U.S. Pat. No. 2,250,912.
A precursor to Assignees downhole motor, which has come to be known as the Dyna-Drill motor is shown in Wallace Clark's U.S. Pat. No. 2,898,087.
Since the rotor of such a motor rotates in an eccentric manner, it is necessary to convert this motion into a true rotation about a fixed axis so that power may be imparted to the drill bit. This can be accomplished by connecting a solid spring type rod between the end of the rotor and output shaft to permit the shaft to rotate about a true axis or by connecting the end of the rotor to a connecting rod by means of a universal joint and connection rod to the output shaft by means of a second universal joint to permit the shaft to rotate about the true axis. A more sophisticated type of universal joint known as the lobed coupling is described in Assignee's U.S. Pat. No. 3,260,069.
It was then found that it was possible to take advantage of the fact that a solid spring type rod or universal joint connection must be present between the turbodrill or the positive displacement type motor and the output shaft and that a clearance space must exist therearound in order to fabricate a bend on the motor housing at this point rather than at the top of the motor. With the arrangement, which was described in U.S. Pat. No. 3,260,318, the location of the bend in the drilling apparatus can be quite close to the bit to provide a larger side load on the bit and to enable one to obtain greater control in orienting the bit.
It was then found that the bend angle practically permissible for a single deviating unit was very limited, in the range of 1 degree to 1.25 degrees. In order to obtain a drilling assembly with a greater degree of bend, a combination of bent sub and bent housing was used. Various types of these assemblies are shown in Assignee's U.S. Pat. No. 4,067,404.
Initially, such bottom hole assemblies were used in an orienting mode only, i.e. the drill string and motor housing was stationary in the hole but oriented in the direction one wished to deviate. After the desired deviation was achieved, the assembly was removed and regular rotary drilling methods were utilized.
However, Assignee later discovered that with strategic location of stabilizers, the drill string with the bent motor housings with or without the bent subs, could be rotated to drill substantially in the straight direction, or the normal orienting mode could be utilized to change angle or make corrections in the bit travel. Such a system, known as the steerable system, is shown in Assignee's U.S. Pat. No. 4,667,751. In such a system a stabilizer is located near the bit.
Although such assemblies are working quite satisfactory, there exists a present day demand for more build rate, i.e. the degree deviation per one hundred feet of drilling. In a single bent housing assembly present day assemblies have still been limited to 1 degree to 1.25 degrees, which would give a build rate of 2 to 6 degrees per 100 feet. In a double bend assembly utilizing a 1.25 degree bent housing and a 1 to 2 degree bent sub, the build rate is about 12 degrees per 100 feet.
A simple solution would be to increase the angle of the bent housing. However, this is not practically possible because of the lack of clearance within the motor connecting rod housing. This increased bending would cause the universal joint to interfere with the housing and damage the motor.
It was also realized that the near bit stabilizer would not operate efficiently with increased bent angles since such a stabilizer would negate build rate.